Dengue virus (DENV) is one of the most significant mosquito-borne viral infections affecting humans today and is an NIAID (National Institute of Allergy and Infectious Diseases) Category A Biodefense pathogen. DENV is a plus-stranded RNA virus and a member of the Flaviviridae family. The four Dengue virus serotypes (DENV1, DENV2, DENV3, and DENV4) are defined by the viral envelope protein (E) and share 60% sequence homology at the amino acid level. Due to the large number of people at risk for infection, DENV is the most widespread mosquito-borne virus affecting humans today. An estimated 2.5 billion people live in areas at risk for epidemic transmission, and an estimated 100 million people are infected with DENV annually. Infection with DENV is responsible for diseases ranging from Dengue fever to the much more severe and life-threatening Dengue hemorrhagic fever (DHF) and Dengue shock syndrome (DSS) that are characterized by vascular leakage. An estimated 500,000 cases of DHF and DSS occur annually and are associated with 2.5% fatality although fatality rates for DHF and DSS can exceed 20% if untreated (Dengue and Dengue haemorrhagic fever. Vol. 2010 (WHO Media Centre, 2010)).
The determinants of disease severity for DENV infection and the pathogenesis mechanisms underlying DHF and DSS are poorly understood. Vascular leakage, which is the hallmark of DHF, is largely believed to be caused by a host-mounted “cytokine storm” but is correlated with high levels of viremia, particularly early in infection (Halstead, Science (1988) 239:476-481; Gubler, Dengue and Dengue Hemorrhagic Fever; Guple et al., Ed.; CAB International: New York, 1997; Vol. 1; pp. 1-22; Vaughn et al., J. Infect. Dis. (2000) 181:2-9; Libraty et al., J. Infect. Dis. (2002) 185:1213-1221; Wang et al., Virology (2003) 305:330-338; and Endy et al., J. Infect. Dis. (2004) 189:990-1000). In vivo data have suggested that therapies that lower Dengue viral burden can ameliorate the inflammatory “cytokine storm” associated with DHF or DSS.
The strong correlation between viral load and disease severity suggests that antivirals that inhibit DENV and reduce viral burden might reduce the severity of DENV-associated disease. Peak virus titers on the order of 107 to 108 MID50/mL (median infectious dose/mL plasma) were found in patients with Dengue fever; patients with DHF or DSS exhibited titers 100-fold and 1000-fold higher, respectively (Vaughn et al., J. Infect. Dis. (2000) 181:2-9; and Libraty et al., J. Infect. Dis. (2002) 185:1213-1221). Additional studies of viremia and estimated infected cell mass confirm this (Libraty et al., J. Infect. Dis. (2002) 185:1213-1221; Wang et al., Virology (2003) 305:330-338; Libraty et al., J. Infect. Dis. (2002) 186:1165-1168; Wang et al., Clin. Infect. Dis. (2006) 43:1023-1030; and Halstead, Dengue. Lancet (2007) 370:1644-1652). Recent experimental support for the potential efficacy of anti-DENV therapeutics includes the demonstration that reduction of virus titers by 70-93% was associated with a reduction in serum levels of inflammatory cytokines (i.e., IL6, TNFα, IL12p70, and MCP-1) in a mouse model of Dengue fever (Schul et al., J. Infect. Dis. (2007) 195:665-674). These effects were observed with 7-DMA, a compound with modest activity against the DENV RNA-dependent polymerase (EC50 15 μM) (Wu et al., J. Virology (2002) 76:3596-3604).
Despite the spread of the four DENV serotypes worldwide and the increasing incidence of DHF and DSS over the past fifty years, there currently are no specific therapeutics to combat DENV infection or a vaccine that protects against all four DENV serotypes. It remains an open question whether immunization directed towards a limited set of epitopes conserved across all DENV serotypes can elicit tetravalent protective immunity or if a successful vaccine will require four distinct type-specific responses. Further complicating DENV vaccine development is the potential for non-neutralizing antibody responses to enhance the infection of immune cells and to exacerbate the disease due to the interaction of the antibody Fc region with activating Fcγ receptors (FcγR) on immune cells, a phenomenon known as antibody-dependent enhancement of infection (ADE). Currently, the only treatment for DHF or DSS is supportive care primarily maintaining the patient's circulating fluid volume. Thus, while continuing efforts to develop a protective tetravalent vaccine are imperative, alternative strategies to prevent and control DENV infection are also urgently needed.